The ultimate objective of our investigations is the rational design of antivirals targeted against human immunodeficiency virus (HIV) structural (Gag) proteins. These proteins are synthesized as domains of the precursor Gag protein, Pr55Gag, which can direct the assembly of immature virus particles. However, normal processing of Pr55Gag yields the mature HIV Gag proteins matrix (MA), capsid (CA), p2, nucleocapsid (NC), pl, and p6, which organize mature virions. The availability of atomic models for matrix, nucleocapsid, and the N-terminal domain (NTD) and C-terminal domain (CTD) of capsid potentially allows the design of ligands which could interfere with the functions of the Gag proteins. However, for this approach to be successful, it is essential to characterize molecular interfaces of the Gag proteins used to choreograph the assembly and maturation of infectious viruses. The focus of our studies is HIV CA which, along with p2 and NC, constitutes the engine of virus particle assembly. We have developed methods for the dissection of HIV capsid interactions, and propose to characterize capsid interfaces in in vitro assemblies, and examine putative capsid targets. The results we obtain will provide crucial insights into mechanisms of assembly and morphogenesis, that should serve as a foundation to attack HIV at its viral core. Our specific aims are as follows: 1. Determination of capsid protein arrangements in particle assemblies: Protein arrangements in in vitro assemblies will be characterized to examine models for how capsid proteins associate in hexamer arrays. The effects of MA and NC domains will be analyzed, and resulting models will be tested in vivo. 2. Evaluation of capsid protein assembly targets: In vitro assembly assays will be employed to test structural models, and to identify accessible capsid residues. These data should prove instrumental in choosing appropriate capsid antiviral targets.